The present invention relates to photoconductive apparatus and methods for generating an electronic signal responsive to selected wavelengths of light. More specifically, the invention relates to apparatus and methods that use photoconductor responses to light in order to provide an electronic output signal corresponding to selected wavelengths. In particular, the apparatus and methods of the invention provide improved wave length specific photosensors with improved sensitivity and responsivity.
Electromagnetic energy is generally thought of as occupying a spectrum (FIG. 1) of wavelengths or frequencies having no inherent upper or lower bounds. The electromagnetic spectrum includes radio waves, x-rays, gamma rays, and the optical spectrum, or light. In general, light occupies a segment of the continuous spectrum of electromagnetic waves from about 10xe2x88x923 meters wavelength, or infrared, up to about 10xe2x88x9211 meters in wavelength, or x-rays. The center of the visible region, or visible light, is at about 555 nanometers in wavelength, which corresponds to yellow-green. Generally, 400 nanometers to 700 nanometers is considered the range for visible light.
Light has a dual nature, it behaves as both a wave and as a particle. A photon is a particle associated with light. Photons can have different energies. When light illuminates a semiconductor, the photons with sufficient energy are absorbed by the material. The electrons from the semiconductor valence band receive enough energy to jump to the conduction band. The conductivity increases due to the higher number of electrons in the conduction band. The electron needs a minimum energy to jump to the conduction band. This minimum energy is the energy gap between the valence band and the conduction band. Photons with energies greater than the energy band gap of the material will be absorbed. Photoconductivity is the increase of current in a semiconductor due to the absorption of photons. A photoconductor converts information and energy from an optical form to an electronic form.
Photodiodes are semiconductor devices that convert light into electrical signals. There are several kinds of semiconductor photodiodes. All work on the principle of photoconductivity. A photodiode has a P-N junction that is reverse biased. Reverse bias means that a positive bias is applied on the N-side of the diode and a negative bias is applied on the P-side of the diode. In the reverse bias P-N junction, no current flows. When an incident photon strikes the P-N junction, it is absorbed and an electron-hole pair is created. The electron and the hole are carried through the junction in opposite directions, creating a current in the photodiode. Photodiodes made from different semiconductor materials are sensitive to different wavelengths of light. Silicon, for example, the most prevalent semiconductor, is sensitive to both visible light and infrared light. Gallium-Arsenide (GaAs) semiconductors are known for sensitivity to both visible and ultraviolet light.
Photodiodes exist in the art for converting various segments of the optical spectrum into electrical signals. For example, visible light photodiodes and ultraviolet photodiodes are known. Difficulties arise, however, in producing photodiodes responsive only to selective wavelengths. Oftentimes the materials used are inherently more sensitive to some wavelengths than others. For example, a serious problem associated with silicon-based visible-light photodiodes known in the art is an unwanted responsiveness to infrared light. One approach to eliminating the infrared response is to use external filters in an attempt to screen out infrared wavelengths. This attempted solution has several shortcomings, important among which are a loss of responsivity in the visible range, increased expense and increased complexity. Another approach to attempting to eliminate unwanted infrared light from visible-light photodiodes is to adjust the wavelengths upon which the device is centered downward, away from the infrared end of the spectrum. This attempted solution results in a loss of sensitivity at the higher frequency end of the visible spectrum.
Photosensitive apparatus and methods able to provide increased responsivity to a particular range of wavelengths without loss of sensitivity would have numerous advantages and uses. Additional advantages, including reduced cost and complexity, would accrue if such apparatus were offered as an integrated unit.
The invention provides apparatus for generating an electronic signal in response to light. A first sensor is provided for converting light to a first electronic signal. A second sensor is provided for converting light to a second electronic signal. A circuit is also provided, for manipulating the first and second electronic signals to generate an output signal responsive to the light.
According to another aspect of the invention, a first photodiode converts light to a first electronic signal. A second photodiode converts light to a second electronic signal. A circuit manipulates the first and second electronic signals to generate an output signal responsive to the light.
According to one aspect of the invention the first and second sensors have a different spectral sensitivity provided by using photodiodes with dissimilar optical thicknesses.
According to the methods of the invention, an electronic signal corresponding to light is generated. The method includes the step of converting light into first and second electronic signals. In another step, the first and second electronic signals are manipulated to generate an output signal corresponding to the light.
Numerous advantages are provided by the invention, including but not limited to reduced response and sensitivity to near infra-red light. The invention also provides a corresponding elimination of sensitivity and responsiveness to deselected wavelengths of light. Advantages of reductions in cost and complexity are realized by the invention in providing an improved integrated wave-light responsive photoconductive apparatus not requiring additional or external components such as filters. These and many other advantages related to the improvements of the invention will become apparent to persons skilled in the relevant arts through careful reading of the disclosure and claims presented herein.